Sirius, the Dog Star, is not only the brightest
star in Michigan's
night sky, it is also the closest one, says U-M
astronomer Richard
Teske.

"Between 9 and 10 p.m. on February nights,
Sirius can be found
almost directly south of us about one-third of the way
up from the
southern horizon," Teske says. "Watch its colorful twinkling.
The
remarkable display is caused by warm currents rising from
buildings
and houses in the frigid winter air which momentarily deflect
the
star's light."

Sirius owes its prominence in our sky to its
nearness to Earth,
Teske explains. Sirius is only 8.6 light-years away
from us. It is
more than 20 times brighter and nearly twice the size of
our sun, but
most astronomers classify it as an ordinary, garden variety
star.
"Many of the stars we see at night are hundreds of times
brighter
than Sirius and dozens of times larger; but they appear
fainter,
because they are much farther away," he says.

Sirius grows
steadily brighter as its distance from Earth
diminishes; the star moves
toward us while sliding off to one side.
"It's as if we stood far back
from the side of a road watching an
automobile approach," Teske explains.
"The star's pace is only
slightly faster than the velocity of the Apollo
astronauts as they
rocketed from Earth to the moon. By the year 61,997,
Sirius will be
closest to Earth---just 7.8 light-years away---after that,
it passes
by and slowly recedes again."

Among astronomers, Sirius'
greatest claim to fame is its strange
companion---the cinder of a
burned-out star. The two are locked in a
mutual gravitational embrace that
causes them to circle one another
perpetually, completing one orbit every
50 years.

Too faint to be seen except with a large telescope,
Sirius'
companion is a type of star astronomers call a "white dwarf,"
because
of its neutral color and size. "White dwarf stars are only about
the
size of Earth, yet each typically contains half as much matter as
the
sun," Teske says. "With so much material packed into such a
small
volume, their density is extraordinarily great. An old textbook
of
mine says that a teaspoon of white dwarf matter would weigh as much
as
seven circus elephants, but I suppose that depends on how one
selects the
elephants."

"Thousands of white dwarfs exist---some of them single
stars, some
that orbit around other stars as does Sirius' partner. They
are the
searingly hot cores of normal stars that long ago ceased to
generate
their own energy and so have died," Teske says. These star cores
shed
their outer layers and slowly cool off, radiating their
remaining
heat into space. Year by year, they glow more feebly, but 10
billion
years or more will be needed before they wink out altogether---a
time
nearly equal to the present age of the universe, according to
Teske.
It is believed that Sirius' companion star became a white dwarf
only
80 million years ago.

The extended periods of time required for
these dense stars to
cool to dark coals is one reason they have attracted
astronomers'
interest, Teske says. "As the corpses of stars that were born
and
died in the early history of the universe, the cinders represent
a
kind of fossil record of the first stars ever
born."